Network Working Group B. Volz
Internet-Draft (Unaffiliated)
Expires: July 5, 2004 January 5, 2004
Reclassifying DHCPv4 Optionsdraft-ietf-dhc-reclassify-options-00
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document revises RFC 2132 to reclassify DHCPv4 option codes 128
to 223 (decimal) as publicly defined options to be managed by IANA in
accordance with RFC 2939. This document directs IANA to make these
option codes available for assignment as publicly defined DHCP
options for future options.
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Internet-Draft Reclassifying DHCPv4 Options January 20041. Introduction
The DHCPv4 [RFC2131] publicly defined options range, options 1-127,
is nearly used up. Efforts such as [unused-optioncodes] help extend
the life of this space, but ultimately the space is expected to be
exhausted.
This document reclassifies much of the site-specific option range,
which has not been widely used for its original intended purpose, to
extend the publicly defined options space.
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Background
The DHCP option space (0-255) was divided into two principal ranges:
1. 1-127 are publicly defined options, now allocated in accordance
with [RFC2939].
2. 128-254 are site-specific options.
Options 0 (pad) and 255 (end) are special and defined in [RFC2131].
3.1 Publicly Defined Options Range
The publicly defined options space (1-127) is nearly exhausted.
Recent work ([unused-optioncodes]) will buy more time as several
allocated but never used option codes are reclaimed. And, a review
could be done from time to time to determine if there are other
option codes that can be reclaimed.
A longer term solution to the eventual exhaustion of the publicly
defined options space is desired. The DHC WG evaluated several
solutions:
1. Using options 126 and 127 to carry 16-bit options as originally
proposed by Ralph Droms in late 1996. However, this significantly
penalizes the first option assigned to this new space, as it
requires implementing the 16-bit option support. Because of this,
options 126 and 127 have been reclaimed [unused-optioncodes].
2. Using a new magic cookie and 16-bit option code format. However,
this proposal:
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* penalizes the first option assigned to this new space, as it
requires significant changes to clients, servers, and relay
agents,
* could adversely impact existing clients, servers, and relay
agents that fail to properly check the magic cookie value,
* requires support of both message formats for the foreseeable
future, and
* requires clients to send multiple DHCPDISCOVER messages - one
for each magic cookie.
3. Reclassifying a portion of the site-specific option codes as
publicly defined. The impact is minimal as only those sites
presently using options in the reclassified range need to
renumber their options.
3.2 Site-Specific Options Range
The site-specific option range is rather large (127 options in all)
and has been little used. The original intent of the site-specific
option range was to support local (to a site) configuration options,
and it is difficult to believe a site would need 127 options for this
purpose. Further, many DHCP client implementations do not provide a
well documented means of requesting site-specific options from a
server or allowing applications to extract the returned option
values.
Some vendors have made use of site-specific option codes that violate
the intent of the site-specific options, as the options are used to
configure features of their products and thus are specific to many
sites. This usage can potentially cause problems if a site has been
using the same site-specific option codes for other purposes and then
deploys products from one of the vendors or if two vendors pick the
same site-specific options.
4. Reclassifying Options
The site-specific option codes 128 to 223 are hereby reclassified as
publicly defined options. This leaves 31 site-specific options, 224
to 254.
To allow vendors that have made use of site-specific options within
the reclassified range to publicly document their option usage and
request an official assignment of the option number to that usage,
the following procedure will be used to reclassify these options:
1. The reclassified options (128 to 223) will be placed in the
"Unavailable" state by IANA. These options are not yet available
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Internet-Draft Reclassifying DHCPv4 Options January 2004
for assignment to publicly defined options.
2. Vendors that currently use one or more of the reclassified
options have until 6 months after this RFC's publication date to
notify the DHC WG and IANA that they are using particular options
numbers and agree to document that usage in an RFC. IANA will
move these options from the "Unavailable" to
"Tentatively-Assigned" state.
Vendors have 18 months from this RFC's publication date to start
the documentation process by submitting an Internet-Draft.
NOTE: If multiple vendors of an option number come forward and
can demonstrate their usage is in reasonably wide use, none of
the vendors will be allowed to keep the current option number and
they MUST go through the normal process of getting a publicly
assigned option [RFC2939].
3. Any options still classified as "Unavailable" 6 months after the
RFC publication date will be moved to the "Unassigned" state by
IANA. These options may then be assigned to any new publicly
defined options in accordance with [RFC2939].
4. For those options in the "Tentatively-Assigned" state, vendors
have until 18 months from this RFC's publication date to submit
an Internet-Draft documenting the option. The documented usage
MUST be consistent with the existing usage. When the option usage
is published as an RFC, IANA will move the option to the
"Assigned" state.
If no Internet-Draft is published within the 18 months or should
one of these Internet-Drafts expire after the 18 months, IANA
will move the option to the "Unassigned" state and the option may
then be assigned to any new publicly defined options in
accordance with [RFC2939].
Sites that are presently using site-specific option codes within the
reclassified range SHOULD take steps to renumber these options to
values within the remaining range. If a site needs more than 31
site-specific options, the site must switch to using suboptions as
has been done for other options, such as the Relay Agent Option
[RFC3046].
5. Security Considerations
This document in and by itself provides no security, nor does it
impact existing DCHP security as described in [RFC2131].
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Internet-Draft Reclassifying DHCPv4 Options January 20046. IANA Considerations
IANA is requested to:
1. Expand the publicly defined DHCPv4 options space from 1-127 to
1-223. The new options (128-223) are to be listed as
"Unavailable" and MUST NOT be assigned to any publicly defined
options.
2. Receive notices from vendors that have been using one or more of
the options in the 128-223 range that they are using the option
and are willing to document that usage. IANA will list these
options as "Tentatively-Assigned".
3. 6 months from this RFC's publication date, change the listing of
any options listed as "Unavailable" to "Available". These options
may now be assigned in accordance with [RFC2939].
4. 18 months from this RFC's publication date and periodically
thereafter as long as there is an option listed as
"Tentatively-Assigned", change the listing of any options listed
as "Tentatively-Assigned" to "Unavailable" if no un-expired
Internet-Draft exists documenting the usage.
7. Acknowledgements
Many thanks to Ralph Droms and Ted Lemon for their valuable input and
earlier work on the various alternatives.
References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
of New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC3046, January 2001.
[unused-optioncodes]
Droms, R., "Unused DHCP Option Codes",
draft-ietf-dhc-unused-optionscodes-07.txt (work in
progress), October 2003.
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